Synthesis, crystal structure and magnetic properties of new trinuclear Copper(II) complexes with Biphenol−Based dinucleating ligands

Treatment of (3,3′-bis-((2,6-dimethylphenyl)-imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H2L2) and (3,3′-bis-((2-methoxyphenyl))imino)methyl)-(1,1′)-biphenyl-2,2′-diol) (H2L3) with excess [Cu2(OAc)4(H2O)2] in the presence of triethylamine afforded new trinuclear complexes [Cu(3,3′-bis-((R)-iminomethyl)-(1,1′)-biphenyl-2,2′-dioxo)]3 [R = 2,6-Me2C6H3 (3e) and R = 2- OMeC6H3(4e)] respectively. The resulting complexes were characterized by elemental analysis, magnetic susceptibility, EPR, UV–Vis, IR, CV and Single crystal X-ray diffraction analysis. The magnetic susceptibility study of 3e and 4e was performed in the 5–300 K and revealed the existence of antiferromagnetic interaction in both complexes. The experimental data could be satisfactorily reproduced using an isotropic exchange model, H = -J (S1S2 + S2S3 + S1S3), yielding as best fit parameters: J = −15 cm−1, g = 1.99 for 3e and J = −18 cm−1, g = 1.99 for 4e. Both complexes have an angular C2/c-symmetric trinuclear core and each Cu(II) ion is in similar environments coordinated to two (O, N) donor sets as revealed from single-crystal X-ray diffraction studies. The coordination geometry can best be described as distorted square planar which could be judged using the τ4 index, τ4 = 360 – (α + β)/141°, where α and β are the two largest angles subtended by the ligand donor atoms in the four-coordinate complex. The average indexes for 3e and 4e were found to be 0.42 and 0.43 respectively, signifying a considerable distortion from an idealized square planar, D4h geometry (τ4 = 0.0) or idealized tetrahedral, Td geometry (τ4 = 1.0). The average Cu⋯Cu distance between closest copper(II) ions in the complex 3e is 4.48 Å. On the other hand, the single crystal X-ray analysis showed that each of the three Cu(II) centers in complex 4e is strongly tetracoordinated to two N and two O atoms & weakly to two O atoms of the methoxy groups of the ligand, N,N′,O,O′-3, 3′-bis ((2-methoxyphenyl)-iminomethyl)-[1, 1′-biphenyl]-2, 2′-diol (H2L3). Generally, Cu−N & Cu−O bond lengths are (1.983–1.994) & (1.882–1.893) respectively, comparable to similar systems.